Sharing of information such as files, data, and the like, sharing of peripheral equipment such as a printer, and the like, or exchange of information such as transfer of E-mails, data contents, and the like, can be realized by connecting multiple computers to form a LAN.
Heretofore, LAN connection has been generally made by a cable such as an optical fiber, coaxial cable, or a twist pair cable, but this case needs a line laid-down construction, which prevents a network from simple establishment, and also complicates lead wiring of cables. Also, even following establishment of a LAN, the movement range of equipment is restricted due to a cable length, which is inconvenient.
Wireless LANs have attracted a great deal of attention as a system for liberating users from LAN cables of the wired method. According to a wireless LAN, the most part of cables in the work space such as an office can be omitted, so communication terminals such as personal computers (PC) can be moved with relative ease.
In recent years, the demand thereof has been markedly increased along with speeding up and price-reduction of a wireless LAN system. Particularly, in these days, implementation of a personal area network (PAN) has been studied for establishing a small-scale wireless network between multiple electronic apparatuses present around a personal environment to perform information communication. For example, different wireless communication systems and wireless communication apparatuses have been stipulated by using frequency bands not requiring the authorization of a competent authority, such as THE 2.4-GHz band, 5-GHz band, or the like.
Examples of the standard specifications relating to a wireless network include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see Non-patent Document No. 1), HiperLAN/2 (e.g., see Non-patent Document No. 2 and Non-patent Document No. 3), IEEE802.15.3, and Bluetooth communication. As for the IEEE802.11 standard, various types of wireless communication method such as the IEEE802.11a standard, IEEE802.11b standard, and the like are available according to the differences of wireless communication methods and frequency bands to be used.
In general, in order to form a local area network using wireless technology, a method is employed wherein one apparatus serving as a control station called an “access point” or “coordinator” is provided within the area, and then a network is formed under totalized control of this control station.
With a wireless network in which an access point is disposed, in the event that information transmission is performed from a certain communication apparatus, an access control method based on a band reservation has been widely employed wherein a band necessary for information transmission thereof is reserved for the access point first, and a transmission path is utilized so as not to collide with information transmission in one of the other communication apparatuses. That is to say, wireless communication is performed synchronously such that communication apparatuses within the wireless network are synchronized one another by disposing the access point.
However, with a wireless communication system including an access point, in the event that asynchronous communication is performed between the communication apparatuses of the transmission side and the reception side, wireless communication always needs to be performed via the access point, and consequently, a problem wherein utility efficiency of a transmission path is reduced by half is caused.
On the other hand, “Ad-hoc” communication wherein wireless communication is performed direct-asynchronously between terminals has been devised as another method for forming a wireless network. In particular, with a small-scale wireless network made up of relatively few clients positioned nearby, the AD-hoc communication is appropriate wherein wireless communication of direct synchronization can be performed between arbitrary terminals without utilizing a particular access point.
An Ad-hoc wireless communication system includes no central control station, so is appropriate for forming a home network made up of domestic electric appliances, for example. An Ad-hoc network has characteristics wherein routing is automatically changed even if one terminal fails to operate properly, or becomes power-off, so is hardly collapsed, and accordingly, data can be transmitted relatively away while maintaining a high-speed data rate by hopping a packet multiple times between mobile stations. As for an Ad-hoc system, various development cases have been known (e.g., see Non-patent Document 4).
Incidentally, under a work environment wherein information devices such as a personal computer (PC) and the like have become widely used, and a great number of devices are mixed within an office, it is assumed that communication stations are scattered, and multiple networks are established in overlapped manner. Under such a situation, in the case of a wireless network employing a single channel, there is no room for retrieving the situation even if another system interrupts during communication, or communication quality deteriorates due to interference or the like.
To this end, with a conventional wireless network system, a method has been generally employed wherein multiple frequency channels are prepared for coexistence with another network beforehand, and one frequency channel to be used in a wireless communication apparatus serving as an access point is selected to start operation.
According to such a multi-channel communication method, when another system interrupts during communication, or communication quality is deteriorated due to interference or the like, network operation is maintained, and coexistence with another network can be realized by switching a frequency channel to be used.
For example, with a high-speed wireless PAN system of IEEE802.15.3 as well, multiple frequency channels available for the system are prepared, a wireless communication device confirms existence of a device which is sending a beacon signal as a piconet coordinator (PNC) to the surrounding area following power being turned on, and accordingly, an algorism is employed wherein a frequency channel to be used is selected by performing scan operation as to all available channels.
With an autonomous decentralized Ad-hoc network in which a control station is not disposed, resource management regarding frequency channels is important to suppress interference with a different wireless network which is running in the vicinity as much as possible. However, in order to switch a frequency channel to be used for the system all at once, a representative station called an access point needs to instruct each terminal station regarding a utility channel. In other words, it is difficult to switch a frequency channel with an Ad-hoc network.
In order to switch between multiple frequency channels, upon HiperLAN/2 being taken as an example, a method for switching the channels all at once has been conceived. For example, an AP (base station) serving as a central control station repeatedly informs MTs (mobile stations) connected to the AP that frequency channels are changed, and the AP and MTs switch the utility channels all at once. Determination regarding whether or not switching should be done is made on the initiative of the AP. Information necessary for this determination can be acquired through the following processing procedures, for example.
(1) According to the instructions of the AP, the MTs which are connecting therewith suspend communication temporarily, scan another frequency channel to perform channel quality evaluation, and inform the AP of the result thereof.
(2) According to the instructions of the AP, the AP suspends transmission of an annunciation channel temporarily, and the MTs now connecting to the AP scan the current frequency channel in use, also perform channel quality evaluation, and inform the AP of the result thereof, and the information is collected by following such procedures.
Also, with Bluetooth communication, a method has been employed wherein each frequency channel is fairly utilized by hopping frequencies with a central control station called a master serving as the basis. In order to form a network, the existence of the master serving as the basis of synchronization between the hopping pattern of a frequency channel and the time axial direction is indispensable. In the event of the master disappearing, the network formed so far once becomes a disconnected state, and the processing for selecting a new master becomes necessary.
Also, with a wireless LAN system of the IEEE802.11 series, a network is formed using the frequency channel set by an access point first, and accordingly, it is difficult to establish an Ad-hoc network without disposing a base station. In the event of performing communication with a wireless communication apparatus (terminal) accommodated in an AP which runs with another frequency channel, between the APs needs to be connected with a wired LAN cable, for example. That is to say, unless between the APs which are accommodated is connected, communication cannot be performed even if wireless communication apparatuses (terminals) which physically adjacently exist are accommodated in a different AP.
Also, with a high-speed wireless PAN system of IEEE802.15.3 as well, it is possible to perform scan of all frequency channels first, and search of coordinators existing in the vicinity, but once implementation using a particular frequency channel is started, the utility situations of other frequency channels cannot be comprehended. Accordingly, even if a piconet using a different frequency channel exists in the vicinity, communication with a wireless communication apparatus connected to the piconet cannot be performed.
Thus, with a conventional wireless communication method, a complex mechanism is necessary, such as timing of switching frequency channels, set-up processing realized by message exchange and the like for terminals, which participate in a network, starting frequency channel switching operation in sync with one another, arbitration processing for determining frequency channel switching, and the like. Also, the existence of a central control station is necessary, such as an AP in IEEE802.11 and HiperLAN/2, or a master in Bluetooth communication, which performs control independently. If the central control station such as an AP, master, or the like disappears, some kind of protocol processing or human-induced setting modification work becomes necessary for selecting a new central control station instead of that central control station, which causes a problem wherein communication is disconnected during that processing.
Also, a wireless communication system has been proposed wherein a frequency channel is determined by measuring not only the interference of the local station channel but also interference using an adjacent channel (see Patent Document 1), but this is a system in which multi-channels are realized by intervening of a base station, so cannot be applied to an autonomous decentralized system.
For example, a method can be conceived wherein a traffic receiving channel is specified by a communication station sending a beacon over the channel most appropriate for the local station. However, even if that channel is most appropriate for the local station, that channel may be a channel which provides interference to a communication station receiving the beacon. It is needless to say that the beacon sending channel which a communication station selected on the basis of the local station is not always a channel which all peripheral communication stations can receive.
Also, in the event that the beacon sending channel of one station is the channel which cannot be used in the other station due to a interference channel or deterioration of communication quality, even if these communication stations can communicate with each other over another channel, these communication stations get trapped in a deadlock state in which they cannot acknowledge one another's existence eternally.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 6-37762
[Non-Patent Document 1]
International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition, PartII: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-Patent Document 2]
ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part1: Basic Data Transport Functions
[Non-Patent Document 3]
ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (RLC) sublayer
[Non-Patent Document 4]
C. K. Tho; Ad Hoc Mobile Wireless Network (Prentice Hall PTR)